Various types of vehicle seat apparatuses have been proposed in the prior art (e.g., patent documents 1 and 2). A seat back of these seat apparatuses is divided into a lower portion, capable of supporting the waist of a vehicle occupant sitting on the seat apparatus, and an upper portion, capable of supporting a blade bone of the vehicle occupant. A seat adjustment device couples an upper frame (first seat member) forming the framework for the upper portion of the seat back and a lower frame (second seat member) forming the framework for the lower portion of the seat back so that the upper frame is tiltable relative to the lower frame in the front to rear direction.
In particular, patent document 1 proposes a seat adjustment device (middle folding mechanism) of which the main component is a link mechanism used so that appearance pivot centers of the lower frame and the upper frame are located in front of a seat back skin that covers a front surface of the seat back frame. As a result, the motion of the seat back when the upper portion is tilted becomes close to the motion of a vehicle occupant when the vehicle occupant bends forward. This suppresses pushing of the back of the vehicle occupant with the upper portion of the seat back, and reduces a sense of being confined or being in a narrow place.
In patent document 2, the upper frame (first seat member) and the lower frame (second seat member) are pivotally coupled to each other by a hinge shaft arranged near a vehicle occupant. A middle folding mechanism couples the upper frame and the lower frame. The middle folding mechanism includes a first link having one end pivotally coupled to the upper frame, a second link having one end pivotally coupled to the lower frame and a further end pivotally coupled to the first link, a drive device, and a telescopic mechanism that may be expanded and contracted back and forth by the drive device. The first link has another end pivotally coupled to the telescopic mechanism, and the further end of the second link is pivotally coupled to the drive device. The telescopic mechanism includes a screw-nut mechanism, for example. In this case, the layout of a coupling point of the first link and the second link, a coupling point of the upper frame and the first link, a coupling point of the lower frame and the second link, a coupling point of the first link and the telescopic mechanism, and a coupling point of the lower frame and the drive device is set so that when a load is input to an upper end of the upper frame, the load applied to the coupling point of the first link and the telescopic mechanism is smaller than the load applied to the coupling point of the upper frame and the first link. As a result, the telescopic mechanism has a structure that is simple and withstands small loads.